In a fixing device of the related art, a sheet is heated by a heater and a toner image on the sheet is fixed by the heat. If sheets having the same width are continuously printed, this causes a situation referred to as excessive temperature rise, in which the temperatures of a heater region located outside a region through which a sheet passes and a fixing belt in contact therewith increase excessively.
If the temperature rise in this non-paper passing region becomes excessive, irreversible performance deterioration such as warpage of a heater, deterioration in a fixing belt, and expansion of conveying and pressing rollers occurs.